Photolithographic processes using photoresists are well known in the art, as discussed, for example, By William S. DeForest in the book entitled "Photoresist Materials and Processes," McGraw-Hill Book Company, New York, 1975. Particular areas in which such photolithographic processes are used are in the formation of metal layers on a substrate to form metal contacts to active regions of an electronic device, in forming optical waveguides and circuits, and in forming reflecting surfaces. Using one conventional process, a patterned metal layer may be formed as follows: (1) a layer of a photoresist material is applied to a substrate; (2) the photoresist layer is irradiated through a mask defining the desired pattern; (3) the photoresist layer is developed (i.e., unpolymerized areas in a negative-acting resist or depolymerized areas in a positive-acting resist are washed away); (4) a layer of the desired metal is deposited over the patterned resist layer, usually by evaporation under vacuum; (5) the patterned photoresist is lifted off, carrying with it corresponding portions of the metal layer; and optionally (6) further processing of the metal layer may be performed. One distinct disadvantage of this conventional process is that during lift-off, the desired metal deposit could also be peeled off, which is particularly a problem when narrow line widths of metal are being formed. In addition, this process requires an expensive and time-consuming vacuum deposition process in order to form the metal layer. Furthermore, this conventional process cannot readily form patterned metal-containing layers in excess of 100 microns in thickness.
Another prior art process by which a patterned metal layer may be formed comprises the following steps: (1) a layer of the desired metal is deposited on the surface of the substrate, usually by evaporation under vacuum; (2) a layer of photoresist material is deposited over the metal layer; (3) the photoresist layer is irradiated through a mask defining the desired pattern; (4) the photoresist layer is developed; (5) an etchant is applied through the patterned photoresist layer to remove the unwanted portions of the metal layer; (6) the photoresist pattern is lifted off; and optionally, (7) further processing of the metal layer may be performed. This process has the disadvantage of requiring time-consuming and expensive evaporation and vacuum procedures and apparatus. In addition, there is the problem of undercutting which occurs when the undesired metal is etched away. Although the desired metal pattern is substantially protected from the etchant by the patterned photoresist, some etchant does flow under the edges of the photoresist pattern and etches away portions of the metal which it was desired to maintain intact (i.e., undercutting occurs). This undercutting is especially a problem when the required width of the lithographed lines is less than the thickness of the photoresist film.
If it is desired to form an unpatterned metal layer on a substrate, prior art methods such as chemical vapor deposition and electron beam sputtering are used and are well known in the art. Both methods require expensive and elaborate apparatus and are time-consuming. In addition, it is difficult to control the composition of the layer deposited by these methods. Similarly, unpatterned layers of dielectric or semiconductor materials have been formed by evaporation, electron beam sputtering, or chemical vapor deposition, which have the latter mentioned disadvantages.
With regard to prior art processes for the deposition of metal from metal-containing compositions in some cases a radiation-sensitive metal salt composition has been used as in U.S. Pat. No. 3,994,727, but no plastic or polymer constituent was used. In another case, where the photoresist as developed consisted of a cross-linked polymer containing metal ions, as in U.S. Pat. No. 3,885,076, the photoresist layer was used merely as a mask for ion-implantation or to form nucleation sites for metal deposition. In still another case, as reported by Robert G. Brault in "Properties of Metal Acrylate Compositions as X-Ray Resists", Electron and ION Beam Science and Technology, Sixth International Conference, (1974), metal has been incorporated into resists for purposes of enhancing a specific physical property of the resist, such as x-ray absorption. However, none of the above prior processes subsequently combusted the organic portion of the photoresist to leave a desired inorganic residue.
In co-pending application Ser. No. 911,543, assigned to the present assignee, an organo-metallic solution is deposited on a substrate and the organic portion is decomposed by heat to leave an inorganic residue, which is then heated to diffuse part of the residue into the substrate. However, the organo-metallic solution, which is defined as having molecules containing a carbon-metal linkage, differs chemically from the metallo-organic photoresist of the present invention, which is described in detail in the discussion of FIG. 3. Further, the layer formed by the decomposition step of the previous process is merely an intermediate step in the process of forming a waveguide device and has no function by itself, as does the layer formed by the present invention.